It was obvious to more savvy comic fans that Kathryn Hahn's Agnes in WandaVision was actually Agatha Harkness from the comics. While the reveal itself didn't catch them off guard, Agatha's incredibly catchy villain song, "Agatha All Along," certainly did.
However, in the finale of WandaVision, it was made a point that Agatha wasn't killed but kept a prisoner in her own mind for whenever Wanda needed her guidance again. A huge indication that Marvel Studios would still have plans for Agatha, especially as Wanda's own story will continue after Doctor Strange in the Multiverse of Madness.
Hahn herself has said that she doesn't know when and where she'll be used next as Agatha, but can't wait to return to the role. The actress was recently asked again about her potential return with a similar answer.
THE (EVENTUAL) RETURN OF AGATHA HARKNESS
In an interview on Gold Derby, Kathryn Hahn was asked again about whether she'd be willing to return to the role, to which Hahn immediately responded, "Of course I'd be willing to come back," but that "I've heard nothing" about any plans Marvel Studios have for her character.
As Marvel Studios President Kevin Feige reaffirmed recently, WandaVision was indeed a limited series, which Hahn was told too, and that the show and role was "a one-and-done situation:"
"I know [head writer] Jac [Schaeffer] and everybody was very clear that this was gonna be a one-and-done situation. You know the season finale was the season finale and written as the season finale, which I think was so brilliant and so satisfied with it as a whole."
Hahn was also asked what aspects of Agatha she would like to explore if she did return to the role, with the biggest appeal to Agatha being "a centuries-old witch" who had seen a lot:
"If I were to ever come back, there's so many aspects of her that I think are fascinating. She touches on so many different worlds throughout the comics. She's a centuries-old witch and has seen a lot. There's a lot of different people and beings that she's crossed paths within the last couple of centuries."
EXPLORING AGATHA'S PAST
Kevin Feige recently said that "the notion of exploring the past, present, and future of the MCU is certainly in the cards for all of our characters," which could include Agatha Harkness. Hahn is correct that the character has an extensive history in the comics and only an implied one in the MCU.
The most fans have seen from the character's past was her attempted execution by her fellow coven witches. It would be interesting to explore how Harkness fell from grace since she's actually a long-time ally to Wanda Maximoff in the comics.
Marvel Studios could also explore New Salem, a magical city hidden in Colorado. A city where Agatha was actually the leader, after being considered the most powerful witch, but would later leave her position as its leader for mysterious reasons.
Something similar could be explored in future films or shows that involve Wanda when she inevitably comes back to speak to Agatha.
Doctor Strange in the Multiverse of Madness is currently scheduled to release in theaters on March 25, 2022.